On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord's Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.
CATECHISM OF THE CATHOLIC CHURCH
Oh, how mistaken in his calculations is he who labors hard on Sunday, thinking that he will earn more money or do more work! Can two or three shillings ever make up for the harm he does himself by violating the law of the good God? You imagine that everything depends on your working; but there comes an illness, an accident so little is required! a tempest, a hailstorm, a frost. The good God holds everything in His hand; He can avenge Himself when He will, and as He will; the means are not wanting to Him. Is He not always the strongest? Must not He be the master in the end?
There was once a woman who came to her priest to ask leave to get in her hay on Sunday. "But," said the priest, "it is not necessary; your hay will run no risk." The woman insisted, saying, "Then you want me to let my crop be lost?" She herself died that very evening; she was more in danger than her crop of hay. "Labor not for the meat which perisheth, but for that which endureth unto life everlasting." [Jn. 6: 27].
What will remain to you of your Sunday work? You leave the earth just as it is; when you go away, you carry nothing with you. Ah! when we are attached to the earth, we are not willing to go ! Our first end is to go to God; we are on the earth for no other purpose. My brethren, we should die on Sunday, and rise again on Monday.
Sunday is the property of our good God; it is His own day, the Lord's day. He made all the days of the week: He might have kept them all; He has given you six, and has reserved only the seventh for Himself. What right have you to meddle with what does not belong to you? You know very well that stolen goods never bring any profit. Nor will the day that you steal from Our Lord profit you either. I know two very certain ways of becoming poor: they are working on Sunday and taking other people's property.